Combined Brayton-Rankine Cycle Thermodynamics

Figure 9-15. Combined Brayton-Rankine Cycle Thermodynamics...

In apphcation to electric utihty power generation, MHD is combined with steam (qv) power generation, as shown in Figure 2. The MHD generator is used as a topping unit to the steam bottoming plant. From a thermodynamic point of view, the system is a combined cycle. The MHD generator operates in a Brayton cycle, similar to a gas turbine the steam plant operates in a conventional Rankine cycle (11). 

The Rankine cycle diagram placed adjacent the Brayton cycle in Figure 9-15 is indicated as a simple steam cycle with superheat, but no reheat and no multi-pressure steam generation. The thermodynamic advantage of the Rankine bottoming cycle is the lowered temperature of heat rejection, in the steam condenser, from the overall combined cycles. 

Determine the mass rate flow of air through the Brayton cycle, and the thermodynamic efficiency and net power output of the Brayton/ Rankine combined plant. Plot the sensitivity diagram of r] (cycle efficiency) versus pn (pressure at state 11). 

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